428 Mr. C. Mereschkowsky on Stauronella. 
a transverse disposition of the chromatophores in the whole 
section of Naviculoid diatoms *. 
If now we compare figures 16-18 of the Plate with the 
diagrams figs. 1 and 3, we can easily see that the endochrome 
of Stauronella constricta belongs to the Nitzschia type, and 
not to the Navicula type. 
There is another difference between the endochrome of 
Naviculacee and Nitzschiew. In all the former the two 
plates (or the two pairs of plates if there are four), if not 
resting on the valves, as in the Punctate { and Lyratew, are 
applied to both connecting-zones (diagram fig. 2). In the 
Nitzschiew both plates rest on one connecting-zone (with 
their margins resting on one or both valves), the other 
zone being free (diagram fig. 4), and this is exactly the 
position which the two chromatophores of Stawronella con- 
stricta occupy (Pl. VIII. fig. 19). Lastly, I could point to 
the conspicuous pyrenoids, which are very rare in true 
Naviculacezs (except in such genera as Cymbella, Gompho- 
mema, &c., which are provided with pyrenoids) and very 
common amongst the Nitzschiee. 
It is only when all these various facts are taken into account 
that the peculiar structure of the endochrome of Stauronella 
constricta acquires all its importance. ‘This species is most 
assuredly not a Naviculoid diatom, its endochrome being 
of the type characteristic of the Nitzschiez. 
But what is it, then? Does it belong to the Nitzschiex ? 
* It may be objected that in Mastogloia, in which, according to Cleve 
(Syn. Navic. Diat. part ii. p. 142), there are only two plates, these latter 
have the same transverse disposition as in Nétzschia, with a line of 
separation along the short axis of the frustule; but in reality this is not 
so. My observations on the endochrome of Mastoglova have shown that 
this genus has four plates, and not two, as stated by Cleve; two of these 
plates belong to the right side of the frustule and two to its left side, both 
pairs being separated by a longitudinal interval. We have therefore in 
Mastogloia the same disposition as in Wavicwa, only with the difference 
that each plate is divided transversely, thus forming in all four plates. 
Some other genera characterized by the presence of four plates (Scolio- 
tropis [Cleve], Neidium, Tropidonets, Pleurosigma, Toaonrdea | Meresch- 
kowsky]) are similarly placed. Although in all these cases there exists, as 
in Nitzschia, a transverse separation in the middle of each plate, dividing 
them in two plates (superior and inferior), yet at the same time both 
pairs of one side of the frustule are separated from the pair of the oppo- 
site side by a longitudinal interval. So that in reality the four plates 
of all these genera (including Mastogloia) have the same disposition as 
the two plates of Navicula, the right plate of Navicula being represented 
by a right pair of plates and its left plate by a left pair. 
“+ I have found the endochrome of the Punctatze to be of the same 
kind as that of the Lyrate. 
